The ubiquitin-proteasome pathway plays an important role in the regulation of cellular proteins with regard to cell cycle control, transcription, apoptosis, cell adhesion, angiogenesis and tumor growth. Proteasome inhibition is a novel approach to the treatment of solid tumors. Bortezomib is the first proteasome inhibitor evaluated in clinical trials. In vitro experiments have shown that bortezomib treatment has a cytotoxic effect on various breast, colorectal, ovarian, pancreatic, prostate, lung and oral cancer cells. Here, the usefulness of proteasome inhibitor for cancer therapy is discussed, and a review of preclinical and clinical studies on proteasome inhibitors alone and in combination with conventional chemotherapy is included.